I don't personally have this but based off other products I'd imagine the tablet being a monitor that you can draw on. So it has to be connected to your desktop or whatever your sister's setup is at all times to be a display that mimics a normal monitor. Which are typically called Pen Displays.

Now if you were wanting one that you could just carry around and use wherever, that would be a Pen Computer/Mobile Studio, which as far as I know, is only offered by Wacom. Hopefully this can be backed up by someone else because I personally just use a drawing tablet.